Storeroom Stories: Petrified Wood

Petrified wood forms from dead trees that have been fossilized. When a tree dies, it is usually broken down by bacteria or eaten by insects. Rarely, a tree may become rapidly buried such as in a landslide or after a volcanic eruption. If this happens, bacteria and insects may not be able to get to the buried tree to break it down. Instead, groundwater will flow through the tree’s tissues, removing the biological components and replacing them with minerals from the surrounding sediment.

This process, known as replacement, causes the tree to have a chemical composition more like that of a rock than living trees. Despite chemically being the same or similar to the surrounding sediment, the petrified wood will keep some of its tree-like physical characteristics, like growth rings. For petrified wood samples that still have their growth rings intact, it is possible to count the rings to determine how old the tree was when it died.